Applied Research On Surface Content And Refractive Index Of Various LiNbO₃ Crystal By Prism Coupler

Many lithium niobate waveguide devices were produced on the surface of lithium niobate, therefore the surface refractive index of lithium niobate crystal and component were very important. The prism coupling technique could measure the surface refractive index simplely and accurately, the surface components (Li₂O content) could be determined indirectly by birefringence. This paper used prism coupling verywhere, studied relevant aspects of the refractive index and component. Including the following three aspects:1. The influence of Li-rich/Li-poor vapor transport equilibration (VTE) on surface Li₂O-content of initially congruent X- and Z-cut LiNbO3 crystal (CLN) plates was studied against VTE duration. The VTE-induced surface Li₂O-content alteration was determined from measured birefringence. The results show that the induced alteration has a square root dependence on the VTE duration for both cases of Li-rich and Li-poor VTE. The dependence shows an effect of cut of plate and is stronger for the Z-cut plate. The Li-rich VTE duration for the surface composition reaching the stoichiometric boundary is similar to 20.0/15.6 h for the X-/Z-cut plate. In contrast, the Li-poor VTE is a much slower process than the Li-rich VTE. The Li-poor duration required to reach the Li-deficient boundary is 400/323 h for the X-/Z-cut plate.2. Contribution of diffused Er3+ ions to refractive index of LiNbO3 crystal and relation of Li out-diffusion to Er3+-diffusion condition have been studied. Three X-cut and three Z-cut congruent LiNbO3 plates locally coated with 10, 15 or 20 nm thick Er metal films were annealed at 1100 or 1130 oC for 60 or 100 h under environmental atmosphere of surrounding air, wet O2 or wet Ar. After the anneal treatment, the ordinary and extraordinary refractive indices at Er3+-doped and Er-free parts of crystal surface were by measurement of critical angle. The difference of surface index values at the Er3+-doped and Er-free parts enables to identify the Er3+-doping effect on substrate index. From the known birefringence, the surface Li₂O contents at the Er3+-doped and Er-free parts were evaluated. The results show that the contribution of Er3+-doping to the ordinary index is within the measurement error of±1×10?3 and that to the extraordinary index is within±1.7×10?3, which is just slightly out of the measurement error. The diffusion temperature is while the diffusion environment is not the key factor responsible for the Li₂O out-diffusion. 3. After the Li-poor VTE treatment of two (6 mol%) Mg-doped LN plates at 1100℃or 1130℃, the birefringence of the plates which was polished layer by layer, was characterized by measurement of critical angle. From the known birefringence, the surface Li₂O contents of the plates were evaluated. Based upon the known depth dependence of surface Li₂O-content, a solution to the diffusion equation, an integral of error function complement, was obtained. Then the diffusion coefficient of Li was calculated by the Boltzmann-Matano method. It showed that there was little difference on diffusion coefficient between Mg-doped LN plate and CLN plate, and diffusion coefficient increased unceasingly along with the temperature increment.